SOLAR EXPERIMENTER
The expanded science behind the activity
Science behind the solar cell
Sunlight that reaches the Earth’s surface is made up, roughly, of 2 equal parts, light we can see (visible
light) and light we cannot see. The part we cannot see is made up of infrared light (heat) and a smaller
amount of ultraviolet light (UV = sunburn). The light we see is composed of the rainbow of colors from
red to violet. Infrared light has longer wavelengths than red light while ultraviolet light has shorter
wavelengths than violet light. Light with shorter wavelengths will have higher energy than light with
longer wavelengths.
Infrared – red – orange – yellow – green –blue – indigo – violet – ultraviolet
A solar cell is a created with special layers of materials that perform different functions. Currently the
most common type of solar cell is created from silicon, a semiconductor element that does not conduct
like a metal nor insulate like plastic. The top layer of the solar cell is a clear coating added to
encapsulate and protect the cell. The next layer is an anti-reflective coating so more light can enter the
cell. The anti-reflective coating gives the solar cell the commonly seen dark-bluish coloration. Under the
coating is a grid of thin aluminum lines. The lines are conductors that collect freed electrons. The
conducting grid is on top of a section of silicon that has been specially treated (doped) to have 3
separate regions. The top negative layer, which has electrons that could be freed to move about, is
above a positive layer, which has places that can accept electrons. At the junction between the two
regions is an area called the N-P junction (negative-positive). This junction creates electrical fields that
encourage any freed electrons to move in one direction, toward the negative layer. On the bottom of the
silicon is an aluminum layer that acts as a conductor for electrons to return to the solar cell.
Clear encapsulating coating
Anti-reflective coating
Grid of thin aluminum conductors
Negative silicon layer
N-P Junction (Negative-Positive)
Positive silicon layer
Aluminum bottom conductor
Clear encapsulating coating
Note illustration thicknesses
are not to scale.
Top view
of grid of
conductors
When visible and infrared light of the correct energy levels reaches an interior layer (the n-p junction) the
light is absorbed and knocks an electron free. The freed electrons were not created - as all the
electrons were always present in the layers of the solar cell. The electron, which had been bound to an
atom, can now move about among the other atoms in the solar cell, ‘bumping’ other electrons out of
place. A grid of thin aluminum lines (wires) on the surface of the solar cell provide a path for the
electrons to push along so some electrons can leave the solar cell. The thin lines cross a slightly wider
aluminum conductor call a “bus” and the bus can be connected to another solar cell or to an outside
connection. The bottom of the solar cell has a layer of aluminum to act as the conductor for electrons to
return (be pushed/pulled back) to the solar cell. The bottom conductive layer is covered with an
encapsulating layer and can be wired to a second, positive, outside connection or to another solar cell.
Solar Experimenter, The expanded science behind the activity, page 1
© 2013, RAFT
The connection to another solar cell within the same unit might be covered such that the connection
cannot be seen. The reason solar cells are connected together is to provide a greater “push” (higher
voltage), just as when a flashlight uses 2 batteries connected in series instead of a single battery. Each
solar cell can produce about ½ a volt so 2 cells connected in series can produce about 1 volt. When 2 or
more solar cells are connected together within the same unit the combination should be called a “solar
panel”. However when the combined unit is rather small, the term “solar cell” is often used.
Insulating tape
(wide black strip)
Series connection
Positive (+)
connection
Solar cells
Negative (-)
connection
Electrical Energy into Motion
The movement of electrons in a wire will create a magnet field around the wire. By coiling the wire round
and round the magnetic fields of the individual wires are combined to make an even stronger magnetic
field. The small motor contains a curved permanent magnet that surrounds a coil of wire on a moveable
center rotor. There are metal contacts connected to the metal tabs at the back of the motor that touch a
part of the rotor that has alternating metal (conducting) and plastic (insulating) sections. The conducting
sections are connected to the ends of the coils of wire. When the tabs at the back of the motor are
connected to a battery or solar cell the movement of electrons in the wires creates a magnetic field that
is repelled by the magnetic field of the permanent magnet. Since the rotor is free to turn the rotor turns,
pushed away by the like magnetic poles and attracted by the opposite magnetic poles of the 2 magnetic
fields. The turning rotor moves the conducting and insulating section of the rotor such that the metal
contacts now touch connections to a different set of coils oriented so the magnetic field will again be
repelled away and attracted towards the permanent magnet’s poles. The process repeats so that the
rotor spins in one direction. If the connections to the battery or solar cell are reversed then magnetic
fields of the opposite orientation will be created. In that case the rotor will spin in the opposite direction.
Resources
Solar cell - http://www.imagesco.com/articles/photovoltaic/photovoltaic.html and
http://library.thinkquest.org/04apr/00215/energy/solar/solar_cells.htm
Energy vs. Wavelength - http://pvcdrom.pveducation.org/sunlight/penergy.htm
Solar Experimenter, The expanded science behind the activity, page 2
© 2013, RAFT